European Patent n. 0873271, filed by the present Applicant, and the pertinent parts of which are included herein purely by way of reference, describes a modular conveyor (known commercially as TTS) comprising a number of modules powered by independent motors, and each of which comprises a contoured structure (e.g. extruded, but which can also be formed using steel sections or other methods) defining a pair of rails, and a powered belt stretched inside the transverse space between the pair of rails, over or under the rails. The modules are arranged with the pairs of rails end to end to form a substantially continuous support and guide structure defining an endless path, and along which run a number of trucks, which engage the rails with respective rollers, and each of which has at least one pair of grip pads for engaging the belt of each module with the aid of push means. The trucks are thus moved selectively by the belt of each module along the rails in a predetermined direction; and transit from one module to the next is made possible by the distance between the pads of each truck being greater than the distance between the belts of two adjacent modules, so that, until the downstream pad of a truck traveling between two modules engages the downstream module, the upstream pad of the same truck still engages the belt of the module the truck is leaving, so that the truck is “pushed” onto the downstream module. Conversely, when the downstream pad engages the belt of the downstream module, the truck is “pulled” by the downstream module belt, thus enabling the upstream pad to leave the upstream module.
Conveyors of the above type have been enormously successful commercially, in that the modules can be floor-mounted or suspended from an overhead structure, and provide for moving the trucks horizontally as well as up and down inclines; curved portions can be formed using modules of appropriate length and shape; and the same trucks can be equipped with floor conveyor pallets, or with attachments, possibly movable vertically, for overhead loads.
For certain applications, however, and particularly for conveying loads along assembly lines, in which a number or “train” of trucks must move at strictly constant speed, with the trucks close together in strictly predetermined positions, the above conveyor is unsuitable, in that, to form a “train” of trucks with the above characteristics, the belt speed of the modules forming the production line would have to be controlled and maintained rigorously constant. Though theoretically possible, using special electric motors and sophisticated electronic controls, the cost involved would be so high as to make the system uncompetitive.
In some cases, therefore, the TTS conveyor is currently unsuitable, in terms of cost, for vehicle assembly lines involving a high degree of precision, and is mainly used for other types of transportation and/or as assembly line return lines.
Assembly lines are therefore equipped with conventional conveyors, in which a number of nonpowered trucks or platforms are accumulated to form a “train”, which is moved along the assembly line by pushing the last upstream platform in the train (here and in the following description, “upstream” and “downstream” are used with reference to the traveling direction of the movable conveying members, be they trucks or platforms) by means of a push station defined by a number of powered rollers, which frictionally and laterally engage the longitudinal edges of the last upstream, i.e. tail, platform in the train; and the train of trucks/platforms is held together, with the trucks/platforms contacting one another, by a downstream braking station acting on the lead truck/platform in the train.
Though possible, the conveyor system resulting from integrating the above conveyor system and the TTS conveyor has several drawbacks, such as relatively high manufacturing cost, excessive rail size, and no standardization. Moreover, the number of trucks/platforms in the “train”, and hence the length of the production line, are restricted by the horizontal force the push station is capable of producing.